How to use structures as properties of a matlab class ?

Dear reader,

I would like to be able to use structures inside classdef and I do now know how to do it. Here is a toy example of what I would like:

classdef dummy1

         properties         
             S.a = 0;
             S.b = linspace(1,2,1e1);
             S.c = [1 2 3];

             P.d = 0;
             P.e = 1;               
         end 

         methods
             %constructor
             function obj = dummy1()
                 obj;
             end 

         end 
   end

Notice the two structures S and P defined inside this class.

However I get an error when running:

dummy1()

Any help is highly appreciated. Once I get through with this, the next challenge would be to have this class ready for code generation.

Thank you

Answer 1

[Guillaume]

The declaration and initialisation of a property in a class must follow the form:

variablename = value;

You can assign a structure as that value, but that structure needs to be fully formed, for example with struct (or cell2struct, or a function that return a struct, etc.):

classdef dummy1
    properties
        S = struct('a', 0, 'b', linspace(1, 2, 10), 'c', [1 2 3]);
        P = struct('d', 0, 'e', 1);
    end
end

[Stefan]

This is indeed a workable example. May I ask you if it possible to write this code alternatively in another way, such that the structure data is allocated inside a method function? Thank you.

[Titus Edelhofer]

Inside the method you use dot notation (again), i.e.

classdef dummy1
  properties
    S
  end
  methods
    function obj = dummy1()
      obj.S = struct('a', 0);
      % or simply
      obj.S.a = 0;
    end
  end
end
end

Titus

[Guillaume]

As Titus said you can create the structure anyway you want, even one field at a time in the constructor. You can also call a function to create the structure from the properties declaration:

classdef dummy1
    properties
         S = createstruct;
    end
end

%local function to create structure 
function s = createstruct
    s.a = 0;
    s.b = linspace(0, 1, 10);
    s.c = 1;
end

[Micke Malmström]

Does this mean that the user extend the structure after creation and add a new part of the structure like below?

obj=dummy1
obj.S.x=1;

[Micke Malmström]

Apparently yes. Then if you want the variables to be "locked" I guess you create a "data"- class like:

classdef SDataClass
  properties
       a = 0;
       b = linspace(1,2,1e1);
       c = [1 2 3];
  end
end

and then let S be such class:

 classdef dummy1
    properties
        S = SDataClass;             % "structure" not editable
        P = struct('d', 0, 'e', 1); % "structure" is editable
    end
 end

[Guillaume]

Another option, if you want to keep the property as a structure, is to validate the structure in the property set method:

classdef dummy1
  properties
       S = createstruct;
  end
  methods
       function this = set.S(this, value)
          assert(isstruct(value) & ...
                 isempty(setxor(fieldnames(value), {'a', 'b', 'c'})), ...
                 'S must be a structure with fields ''a'', ''b'', ''c'' exclusively');
          this.S = value;
       end
  end
end

[Leon Stolp]

I am trying to achieve the same thing, but for a dependent property.

 properties (Dependent)
        
        Numbers = struct (Fn, Rn)
        Coeff = struct (C_B, C_WP, C_P, C_M, C_F)
        S_BH
        R_F
        ff = struct (C_14, L_R, formfactor)
        Rapp = struct (APP_factor_eq, R_APP) 
        Rw = struct (C_3, C_2, C_5, C_15, m_4, lambda, C_16, m_1, R_W) 
        Rb = struct (P_B, Fn_i, R_B)
        Rtr = struct (Fn_T, C_6, R_TR)
        Ra = struct (C_4, C_A, R_A)
        R_T
    end

However, I get a "error" that tells me, that Default values should not be assigned to Dependent properties. It's causing further Problems, as I can't access the structs later on in the methods. Is there a way to make it work?

[Ludo Houben]

@Guillaume how to change the code for 'S' in case you want to use a simulink.bus definition called 'testbus'

Simulink.Bus: testbus

Simulink.Bus.Element: 'value1'

Simulink.Bus.Element: 'value2'